1. Field of the Invention
The present invention relates to an arc discharge apparatus used to heat a silica powder by arc discharge and cause it to fuse and vitrify, an apparatus comprising the arc discharge apparatus for manufacturing the vitreous silica glass crucible, a method for manufacturing the same and a method for pulling up a silicon single crystal.
2. Description of the Related Art
A vitreous silica glass crucible used in producing a silicon single crystal by pulling up is manufactured mainly by an arc fusing process. With this process, a silica powder is placed as a layer of a predetermined thickness on the inner surface of a mold formed from carbon, generating arc discharge between a plurality of carbon electrodes disposed above the deposited silica layer, and heating the deposited silica layer so as to fuse and vitrify, thereby manufacturing the vitreous silica glass crucible.
With the manufacturing process described above, a part of the silica powder heated to a high temperature melts and gasifies to produce silica fume when the silica powder is fused by the arc. This method has such a problem that the silica fume deposits on the electrode surface and the coagulated silica fume falls into molten fused silica (dropping phenomenon), thus resulting in deposition of a foreign matter on the inner surface of the vitreous silica glass crucible or inhomogeneity of the fused silica.
There is also such a problem that, when a carbon electrode that is not sufficiently homogeneous is used, the arc becomes uneven which may result in chipping of the electrode, thus causing carbon chips that come off the electrode to deposit on the surface of the vitreous silica crucible and undergo incomplete burning, resulting in the production of black foreign matter. Even when the black foreign matter is completely burned, pits are formed on the crucible surface resulting in an undesirable state of the surface. In particular, when the crucible is regenerated, since arc discharge is generated with less power and of a shorter duration than those employed during manufacturing the crucible so as to avoid deformation of the crucible, carbon chips coming off the electrode are more likely to undergo incomplete burning and produce black foreign matter.
Carbon particles that constitute the carbon electrode are gradually consumed as the carbon particles residing on the surface are burned by the arc. Burning carbon particles, when small in size, burn out before reaching the crucible surface. Large carbon particles, however, do not burn out before falling onto the crucible surface, with the remnant turning into black foreign matter or burning on the inner surface of the crucible to form pits on the surface. Such black foreign matter and the irregular inner surface of the crucible decrease the yield of producing the single crystal when pulling up a silicon single crystal.
To solve the problems described above, Patent Document 1 (Japanese Unexamined Patent Publication (Kokai) No. 2001-97775) discloses a carbon electrode formed from carbon particles having a maximum carbon particle diameter of 150 μm or less, an electrode density of 1.80 g/cm3 or more and a strength of 35 MPa or more as measured by a 3-point bending test.
Patent Document 2 (Japanese Unexamined Patent Publication (Kokai) No. 2002-68841) discloses a high-purity carbon electrode for arc fusing application formed from carbon particles having a particle diameter in a range of from 0.05 to 0.5 mm.
The carbon electrode disclosed in Patent Document 1 has such problems that molding extremely small carbon particles into an electrode of high density and high strength is expensive, and that unevenness in density of the electrode makes the arc unstable and may cause chipping of the electrode. Moreover, when the density of the electrode is too high, the carbon particles bond with each other too strongly. As a result, lumps of carbon particles are scattered as the electrode is consumed during arc discharge, and the lumps that fall onto the inner surface of the crucible without burning out may produce black foreign matter and/or pits.
The high-purity carbon electrode disclosed in Patent Document 2, on the other hand, has economical advantages but needs to be improved in terms of the uniformity of electrode density and in terms of particle diameter of the carbon particles.